While nosing around the lower area of the Flophouse (next to the cave from the Remnant Drive Core quest), activate the exclamation'd console to overhear a conversation about Annea's water supply.
Back at Paradise Sands, check out Annea's office, above the store.
Trace the power cable back to a console at ground level, southeastish of the store.
Enter the office and read a memo about her secret underground lake. Note that you cannot loot the big container here, at least not yet. Exit, and head to your new navpoint, far to the northeast.
Enter the door here, and follow your navpoint through the cave. Scan the Remnant device in the water.
Annea shows up with some goons, and now you have a choice to either keep her secret or not. If the former, she stays where she is, and you can check that container in her office for regular loot drops. If you refuse, there's a fight, and she flees after pledging to hound you forever.
There are few lasting effects (in this game), but a bit of a tough call. On the one hand, Annea isn't the most benevolent monopolist you'll ever meet, and the colonies here are, to an extent, living on her forbearance. If she's driven off, she'll be replaced by a (presumably more just) Nexus representative. On the other hand, it is her home, her find, and the only reason she keeps it secret is to prevent other people from killing her and stealing it. Kicking someone out of their home because you want their stuff isn't a great look either. Besides, you've got the ice-pushers of Voeld, Remnant tech in the works, and there may even be other lakes here. Whatever you decide, this quest ends when you check back in at The Paradise, either to collect your tithe from Annea, or greet the Nexus rep.
Note that sometimes the tithe box just stops working- you can activate it, but get no loot.